The present invention relates to an image data processing unit that converts image data, on the basis of the conversion data stored in a look-up table.
A charged particle beam instrument which like a scanning electron microscope has a scanning function, allows observation and photographing of specimen images. In such an instrument, secondary electron signals are obtained by scanning the specimen surface with a charged particle beam. After being amplified, those signals are converted to digital signals and then stored in a memory as image data. And, as occasion demands, the image data is read out of the memory and sent into a display unit such as a cathode ray tube. A specimen image is thus displayed on the screen. Such a scanning electron microscope is disclosed in U.S. Pat. No. 4,236,178.
When it is desired to adjust the contrast, brightness, or gray scale gradation of a specimen image displayed on the display unit to a level suited for observation, the image data from the memory is changed to values suited for the desired contrast, brightness, or gray scale.
FIG. 1 is the block diagram of an image data processing unit that carries out such image data conversion. In the diagram, a buffer memory 1 stores digital image data temporarily. A look-up table 2 stores the conversion data needed for executing data conversion. Look-up table 2 is used to subject the image data supplied from the buffer memory 1 to the desired conversion. A control system 3 gives the look-up table 2 a command as to what type of data conversion is to be carried out. A digital-to-analog converter 4 converts the converted image data to an analog signal. The image data that was converted to an analog signal by the DA converter is supplied to a display unit 6 like a CRT via an amplifier 5. This causes a specimen image based on the converted image data to be displayed on the screen of the display unit.
In the look-up table 2 are stored conversion data on single display characteristics only, such as conversion data A on the contrast, conversion data B on the brightness, and conversion data C on the gray scale. Furthermore, in the look-up table 2 are also stored composite conversion data on plural display characteristics, such as conversion data AB that combines the contrast and brightness, conversion data AC that combines the contrast and gray scale,, and conversion data ABC that combines the contrast, brightness and gray scale. The conversion data stored on the look-up table is selected by a command from the above-mentioned control system 3 in accordance with the purpose of image display. The image data from the buffer memory 1 is converted on the basis of the conversion data selected from the look-up table 2.
In an instrument of the above construction, if the types of conversion data related to a single display characteristic are numerous, the types of composite conversion data related to plural display characteristics also become numerous. This makes it impossible to store all necessary data in the conversion data storage area of the look-up table 2, thus making it impossible to completely carry out the desired data conversion. And, as the number of combinations of display characteristics increases, the creation of composite conversion data becomes more difficult, resulting in increasing the cost for creating the conversion data.